Touch screens allow a user to conveniently interface with an electronic display system. For example, a user can carry out a complicated sequence of instructions by simply touching the screen at a location identified by a pre-programmed icon. The on-screen menu may be changed by re-programming the supporting software according to the application.
Resistive and capacitive are two common touch sensing methods employed to detect the location of a touch input. Resistive technology typically incorporates two resistive films as part of an electronic circuit that detects the location of a touch. Capacitive technology, on the other hand, typically uses a single resistive film to detect the location of an applied touch.
A touch location is generally determined by applying an electric field to a resistive film in the touch sensitive area. Where the transparent conductor is an electrically continuous coating in the touch area, the accuracy of detecting the location of an applied touch depends on the linearity of the electric field in the transparent conductor.
Various methods have been proposed to linearize the electric field. For example, in a four wire resistive touch technology, a pair of highly conductive continuous electrode bars are formed onto a resistive film at two opposite edges of a touch sensitive surface. A differential voltage applied to the two conductive bars results in a fairly linear electric field in the plane of the resistive film in the direction normal to the two electrode bars. Similarly, a second pair of highly conductive electrode bars are formed on a second resistive film with the bars being orthogonal to the first pair of bars.
As another example, five wire resistive or capacitive touch sensors typically employ an electrode pattern along the perimeter of a touch sensitive area to linearize the field. In a five wire resistive touch sensor, a second transparent conductor typically acts as a current sink or voltage probe and does not require linearization. In a five wire capacitive touch sensor, a user's finger or other conductive implement may provide the current sink. The electrode pattern is typically made up of a number of discrete conductive segments positioned in such a way as to generate a linear orthogonal field in the plane of the transparent resistive film.
Typically, the linearizing electrode pattern includes several rows of discrete conductive segments positioned along the perimeter of a touch sensitive area, such as disclosed in U.S. Pat. Nos. 4,198,539; 4,293,734; and 4,371,746. The conductive segments are typically electrically connected to each other via a resistive film they are deposited on. U.S. Pat. No. 4,822,957 discloses rows of discrete electrodes having varying lengths and spacings to linearize the electric field in a touch area.
Several factors can determine the efficacy of a linearization pattern. One such factor is the degree to which the field can be linearized. Some electrode patterns may be incapable of linearizing the field to a level required in a given application.
Another factor is the end-to-end resistance of an electrode pattern, which can be measured, for example, in a rectangular electrode pattern, by applying a voltage to the two corners of one edge of the pattern, and applying a different voltage to the two corners of the opposite edge, and measuring the current that flows between the two edges. A lower value of end-to-end resistance in an electrode pattern typically yields better linearity. A lower end-to-end resistance, however, can increase signal drive requirements and may reduce device sensitivity. Accordingly, a high end-to-end resistance is often desirable when designing an electrode pattern.
Another factor is sensitivity of field linearity to small variations in the electrode pattern. Such variations are typically unavoidable during manufacturing. If small variations in the electrode pattern result in unacceptable nonlinearity in the electric field, the yield and hence the cost of manufacturing a touch sensor may be adversely affected.